by L. MEULSTEE, PAHPCR.

A historical and technical description of a Servicesignal instrument, invented in 1915 by Captain A.C.Fuller and still in use during and after WW2.

1915... two large armies were densely packed in theirtrenches, at places only a few hundred yards apart.Signal communication was mainly by telephone and variousbuzzer telegraph instruments, connected via single cableand earth return. The earth was thus alive with buzzerand telephone induction.

During mid-1915 the Germans were extraordinarily wellinformed of Allied plans. Carefully planned raids weremet by hostile fire exactly timed and directed. Relievingtroops would be greeted, if not by shells, by shouts ofwelcome from the opposing trenches.

On one occasion a Scottish battalion took over its newfront to the strains of its regimental march played bya German cornet! Espionage was suspected but an internedBritish civilian brought back the information that induct-ion of cables ledto widespread interception of signals.

Experiments carried out within the Allied lines leftno doubt of the cause of the leakage, and measures againsteavesdrop ing were hastily introduced by using metalliccircuits two twisted wires), instead of an earth return,

within 4000 yards of the front—line.The solution to the problem came toward the end of

1915 when Captain (later Major General) A.C. Fullerinvented the Fullerphone, a small direct current Morsetelegraph instrument. In October 1915, Fuller broughttwo prototypes to 5 Corps in Flanders. His inventionwas tested on a five mile loop of cable, part of whichran in the water-filled moat of Ypres, with a 10 ohmleak to earth. The instruments worked well and wereobviously the answer to the problem of interception byinduction which had brought the BEF signals systemalmost to a standstill.

Fig. 1. Fullerphone MkV, tropicalised version of theWN2 standard instrument. The control panel carriesthe line terminals, potentiometer control knob,reversing switch, and phone sockets. The Morse key ismounted at the bottom right-hand side of the unit. Thebuzzer—interrupter (chopper) unit slides into positionon the top right-hand side.

Break-in ossibleThe Fullerphone is essentially a d.c. Morse teleg-

raph instrument with high sensitivity. Morse signalscan be sent and received by the same instrument; nosend-receive switch is used, so 'break—in' working ispossible. When sending, a very small direct currentwill flow through the line and the receiving instrument.A readable signal will be produced with a line currentof only 0.5 microamp. For reliable communication, how-ever, a current of 2.5 microamp is considered essential.

To make the d.c. Morse signals audible, Fuller addedan interrupter device to change the steady current intoan intermittent current suitable for producing a notein the headphones. He completed the circuit by insert-ing capacitors and l.f. chokes to keep the current inthe line constant as long as the key is down. The curr—ent from the sending battery passes through the sendingoperator's 'phones, enabling him to hear his own buzzersignals and to judge the strength of signals to the line.

transeitting static1 receiving station

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buzzer signals. Should his own interrupter go out ofadjustment, he will hear nothing at all.

The arrangement of chokes and condensers not onlyprevents any appreciable variation in the line current,but also prevents any l.f. currents (such as producedby induction from other circuits) from passing through

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Fig. 3. Circuit diagram of Mk IV Fullerphone (1939).

the operator's headphones.The effect of the capacitors and self—inductance of

the coils also prevents the possibility of Horse signalsbeing read from clicks by the enemy, or clicks interfer-ing with telephony carried over the line simultaneouslywith Morse signalling.

Fullerphone signals cannot be overheard by inductionor earth leakage, and can only be tapped by the directconnection of a similar instrument to the line. Working

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Fig. 4.cettabile — Mod. 1931".non-interceptable).phone in the early thirties.the British design, but no line balancing potentiometeris included.

"Apparado da campo per telegrafia ininter—(Apparatus, field-telegraph,

The Italian Army copied the Fuller-The circuit is similar to

via leaky or very long cables is possible, the normalrange for reliable communication being 25-40 miles. Muchgreater ranges are possible under special conditions (eg,air lines in the desert) or by putting in a minor circuitchange. As the Fullerphone works on d.c. only it can beused on one line simultaneously with a telephone setwithout any mutual interference.

DifficultiesFullerphone circuits are, however, liable to some

forms of interference. Difficulties in working are almostinvariably due to interference from small currents pickedup by the line, either by the earth, by earth faults, orby leakage from other currents. The interference causedby a steady earth current, or by leakage currents fromother circuits, can be effectively balanced out at thereceiving end by producing a current of equal strengthbut of an opposite direction to the interfering current.A potentiometer R, dry cell B2, and reversing switch 32are provided for this purpose. Each station must adjustits own potentiometer as the currents which are pickedup as earth faults will not necessarily be the same ateach end of the line. '

Fig. 5. 1‘0ka 1942... Mk IV Fullerphones in use atAustralian Signal Headquarters in the desert.

Used in two warsInitial issues of Fullerphones were made up from

converted DIII field telephone sets. This type, however,was not the most successful. Toward the end of 1916, theFullerphone was firmly established, and by 1918 mostDivisions had adopted Fullerphones for all their forwardcircuits. After the armistice, improvements and modific—ations to the instrument were carried out. The basicprinciple, hOWever, was never changed. During 1939 a

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i w rFig. 6. Arrangement for cables up to 500—600 miles.Alterations to the standard Mk IV Fullerphone for thisapplication included an increase of voltage on the linefrom 1.5 to 12 volts.

re—designed model, Mk IV, went into service. This wasmore sensitive than its predecessors, with a more easilyadjustable buzzer/interrupter, and was simpler to use asit carried no telephone set.

The Fullerphone was designed as a non—interceptablesignal instrument for static trench-warfare. But it waswiiely used during WW2 because of its other features,vi , the ability to work simultaneously with a telephoneon the same line, and the capability of working throughvery long or leaky lines where telephone traffic wasimpossible.

In the South West Pacific, for example, the Austral-ians made extensive use of the Fullerphone, and it morethan lived up to its reputation when used in New Guinearesulting in a considerable saving of cables at a timewhen men and supplies were scarce resources.

During WW2 cases arose where a submarine cable circuitwas available but the necessary terminal telegraph equip-ment was found to be totally destroyed or not immediatelyavailable. To ascertain to what extent Fullerphonescould be used on submarine circuits of various lengths,trials were carried out, at the request of the War Depart—ment, by Cable & Wireless Ltd. The results exceeded allexpectations, and ranges of up to 700 miles were obtainedwith faint but readable Morse signals at a maximum of20 words per minute.Finally

The attitude of signallers to their Fullerphonescannot be better described than in a poem written bySignalman R. Mellor and published in "Jimmy", the mag-azine of the Royal Signals in the Middle East.

@1315, T0 A. FULLERPHQNE\JKZhat is my greatest joy in life,

More precious even than my wife,So comforting ‘midst all this strife?

My Fullerphone.How well I love your merry tricks;Even when your buzzer sticks;Delighting me with faint key clicks;

Oh Fullerphone.How tunefully your buzzer throbsAs tenderly I turn those knobs.Most fascinating of all jobs.

0h Fullerphone.

Potentiometer, its trueI'm not sure what to do with you.Yet even you add beauty to

My Fullerphone.Oh how I pity those poor soulsWho daily work remote controls,Attached to crazy wireless poles.

Oh Fullerphone.They never hear the tuneful tonesOf perfect Morse within their 'phones:Just atmospherics, shrieks and groans.

0h Fullerphone.But I must cease to write more verse.Communication's getting worse.No wonder that I rave and curse

At Fullerphone.Asthmatic buzzers, - crazy keys.How can one live a life of ease,With damfool instruments like these

Foul FullerphoneslSigmn. R. MELLOR

AcknowledgementsThe author is indebted to Major A.G. Ear-field, 33.11.,Deputy Director of the Royal Signals Museum, for his helpand permission to publish the photographs and diagrams inthis article.The Royal Signals Museum at Blandford Camp, near Bland-ford Forum, Dorset, is open to the public during weekdaysfrom 8.30 to 1600, and a collection of Pullerphones isamong many other signal instruments.